From WO 2007/090011 A2 a generic device is known. It comprises an air inlet duct that extends between the exterior surface of the helicopter and the engine, which is arranged on the interior, which air inlet duct comprises an inlet-cover filter element that is flush with the exterior surface of the helicopter. In this arrangement the inlet-cover filter element follows the contour of the exterior surface and comprises an exchangeable flat filter body that is held by a closed attachment frame made of metal, and that on both sides comprises a protective grille. The inlet-cover filter element filters the air from the environment, which air is ingested by the engine, in order to stop foreign objects and dirt particles from entering the engine by way of the air inlet duct. After a certain period of time the filter body of the inlet-cover filter element is usually blocked and requires cleaning. Thereafter it can again be placed into the attachment frame. The attachment frame is used to ensure the desired shape of the filter body and thus its function even at high rates of flow of air from the environment.
In the implementation of an inlet-cover filter element for the engines to be protected, for reasons associated with safety, it must always be ensured that the icing behaviour on the filter bodies and on the air inlet duct is kept within permissible limits. The installation of an inlet-cover filter element that is used to prevent premature engine wear as a result of erosion is always a far-reaching intervention in the conception of the air inlet duct. As a rule, only a single air inlet duct will be available for feeding combustion air to an engine. In such a single engine inlet, performance loss due to the pressure loss resulting from the filter body that is installed upstream will practically be unavoidable. Dangerous hazard conditions arise in particular when icing of the filter body results in total air impermeability.
In U.S. Pat. No. 4,190,217 a device for the supply of combustion air to an engine of an aircraft is stated, in which device the above-mentioned problem is solved in that in the region of the inlet-cover filter element an additional inflow flap is provided. If the inlet-cover filter element is clogged, then this inflow flap opens in order to lead a replacement airstream, which will then, however, not be filtered, to the engine. This emergency measure is associated with a disadvantage in that engine wear cannot be avoided. If the above-mentioned emergency situation arises, and if at the same time the air from the environment is severely contaminated with coarse dirt particles, emergency operation can directly lead to situations in which the engine sustains considerable damage.
From prior art it is also known, instead of providing an inflow flap that is arranged beside the inlet filter element, to design the filter element itself so that it is tiltable so that the gap forming between said filter element and the exterior surface of the aircraft makes possible an emergency supply of non-filtered combustion air. Most of the time this results in severe inlet losses to the engine. Since it must always be ensured that the available performance exceeds the maximum weight of the helicopter, this inlet loss is a limiting factor.